Transcript How Things Work

Carousels and Roller Coasters 1
Carousels and Roller Coasters
Carousels and Roller Coasters 2
Question:
• You are a passenger in a car that is
turning left and you find yourself thrown
against the door to your right. Is there a
force pushing you toward the door?
Carousels and Roller Coasters 3
Observations About
Carousels & Roller Coasters
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You can feel motion with your eyes closed
You feel pulled in unusual directions
You sometimes feel weightless
You often can’t tell when you’re inverted
Carousels and Roller Coasters 4
The Experience of Weight
• When you are at equilibrium,
– a support force balances your weight
– support force acts on your lower surface
– your weight is spread throughout your body
• You feel internal supporting stresses
• You identify these stresses as weight
Carousels and Roller Coasters 5
The Experience of Acceleration
• When you are accelerating,
– a support force causes acceleration
– support force acts on your surface
– your mass is spread throughout your body
• You feel internal supporting stresses
• You misidentify these stresses as weight
Carousels and Roller Coasters 6
Acceleration and Weight
• Acceleration produces fictitious “force”
– Not a real force at all
– Just a feeling caused by your body’s inertia
– Directed opposite your acceleration
– Proportional to the acceleration
• Gives rise to “apparent weight”
– Feeling of real weight plus fictitious “force”
Carousels and Roller Coasters 7
Carousels, Part 1
• Riders undergo “uniform circular motion”
– Riders follow a circular path
– Riders move at constant speed
• UCM involves centripetal acceleration
– Acceleration points toward the circle’s center
– Depends on speed and circle size
Acceleration = velocity2 / radius
Carousels and Roller Coasters 8
Carousels, Part 2
• Centripetal acceleration requires
– force directed toward circle’s center
– This centripetal force is a true force
• Centripetal acceleration yields
– a fictitious “force” called “centrifugal force”
– “Force” is directed away from circle’s center
– An experience of inertia, not a real force
Carousels and Roller Coasters 9
Question:
• You are a passenger in a car that is
turning left and you find yourself thrown
against the door to your right. Is there a
force pushing you toward the door?
Carousels and Roller Coasters 10
Roller Coasters, Part 1 – Hills
• During hill descent,
– acceleration is downhill
– fictitious “force” is uphill
– apparent weight is weak and into the track
• At bottom of hill,
– acceleration is approximately upward
– fictitious “force” is approximately downward
– apparent weight is very strong and downward
Carousels and Roller Coasters 11
Roller Coasters, Part 2 – Loops
• At top of loop-the-loop,
– acceleration is strongly downward
– fictitious “force” is strongly upward
– apparent weight is weak but upward!
Carousels and Roller Coasters 12
Choosing a Seat
• As you go over cliff-shaped hills,
– acceleration is downward
– fictitious “force” is upward
– higher speed  more acceleration and “force”
• First car goes over cliff slowly
• Last car goes over cliff quickly
• Last car has best weightless feeling!
Carousels and Roller Coasters 13
Summary About
Carousels & Roller Coasters
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You are often accelerating on these rides
Feel fictitious “force” opposite acceleration
Your apparent weight isn’t always down
Your apparent weight can become small
Your apparent weight can even point up